This may not matter to you if you only FaceTime from your iPhone occasionally, but if you're FaceTiming every day on a limited data plan from your cellular provider, you're going to want to conserve as much data as possible by making sure that you're connected to Wi-Fi only — especially if they are video calls.
Older versions of FaceTime made it super easy to prefer Wi-Fi over a cellular connection straight from FaceTime's preferences in the Settings app. In recent iOS versions, though, it's not as easy to find, but it's still fairly simple.
Before making FaceTime a Wi-Fi only app, it's good to verify just how much data FaceTime is consuming on your iPhone. If you're only wasting 100 MB every month, you might not want to read any further. If you're wasting 500 MB or more on a 1 GB or 2 GB data plan, then you'll probably want to go Wi-Fi only after all.
There are a couple ways to see how much data your FaceTime calls are using. First, open up your FaceTime app, then hit the "i" (info) button next to your calls. On the next screen, it will tell you how much data each call took. Alternatively, you could also visit the "Recents" tab in the Phone app and do the same; Just look for the "FaceTime Video" or "FaceTime Audio" indicator next to the calls in the list.
You'll probably notice that audio calls barely use any data and that FaceTime video is the real culprit here. So, if you only use FaceTime audio because you like the quality compared to regular cellular non-data calls, then you might not care about data usage. But if you're a FaceTime video fanatic, you'll want to continue. ...
If you decided to ax cellular calls for FaceTime, open up your Settings app and tap on "Cellular." Next, scroll down the list of app names until you see "FaceTime," then make sure it's toggled off.
You'll also notice here that it shows you data usage for each app, but don't take this to heart as it's not based on your monthly cellular plan, but from the last time you "Reset Statistics" at the bottom of this page.
That's all there is to it. Really. Now every time you make or receive an audio or video call using FaceTime, you will need to be connected to a nearby Wi-Fi network.
If no Wi-Fi hotspots are available and you must FaceTime, you can go back into these settings and toggle it back on for emergencies. Since your goal here is to save on cellular data, making you work for it when you have to actually use data is the best way to keep you from using it when you don't have to.